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“Big Blue Marble” image taken on 7 Dec. 1972 by the crew of Apollo 17.

Forty years ago, the crew of the last Apollo mission took this photograph of Earth during their return flight from the moon. This is one of the most important photographs ever taken. In 1948 British astronomer Fred Hoyle stated that a photo of the earth from space would generate “a new idea as powerful as any in history.” Hoyle was right: that photo reinforced the growing ecology and environmental movements. But its most important long-run effect is that it has expanded the scale and scope of how humanity thinks about its place in the universe.

I rate the chances at about 50% that we will find life elsewhere in the universe by the end of this century. In my opinion, this is the most pressing scientific question facing humanity. We are actively trying to answer that question along two directions, and our efforts will likely accelerate over the course of this century.

The first direction is to explore our own backyard. We first placed an extended-mission vehicle on another planet in 1997, the Mars Pathfinder mission. Since then, there’s been a lot of hardware placed in Mars orbit or on the planet itself. The latest mission, Curiosity, has sensed organic molecules in Martian soil, though scientists are not certain that this indicates microbial life. Within this century, it’s likely that American, European, Japanese, and other nations’ space agencies will thoroughly explore Mars, and even other bodies like Jupiter’s and Saturn’s moons. By 2200 we should have a pretty good answer to the question of whether life exists elsewhere in the solar system, and if there is a common origin for it.

The more interesting question is whether life exists outside our solar system. The first confirmed discovery of an extrasolar planet was in 1995. Thanks to the Kepler space telescope, we have confirmed the existence of nearly a thousand of them with another two thousand candidate planets awaiting confirmation. Follow-on space telescopes are poised to discover tens of thousands of extrasolar planets and to extract enough signal to get a rough guess at their atmospheric composition. The presence of water vapor, oxygen, carbon dioxide, and methane would be a pretty good indicator of life with a similar chemistry to ours.

Then what? Let’s say we find extrasolar planets with high potential for bearing life. Do we have the scientific and technological expertise to send a robotic probe across several light years to investigate? If such a mission takes a hundred years or more, can we plan that far ahead? And most importantly, how will discovering life around another star change us?